Fix GFS2's need_sync()'s use of do_div() on an s64 by using div_s64() instead.

This does assume that gt_quota_scale_den can be cast to an s32.

This was introduced by patch b3b94faa.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>